Apparent Magnetisation from Aeromagnetic Data

Abstract

Magnetic effects associated with the terrain may cause great difficulty in the interpretation of aeromagnetic surveys, especially for draped surveys above a rugged magnetised terrain. A method has been developed that allows (gridded) magnetic field observations and topography (described by a digital terrain model) to be inverted to obtain a map showing the apparent magnetisation of the surficial rocks. The surficial rocks are modelled as either a surface shell of given thickness, or a layer with its upper surface defined by the topography and a flat lower surface of given elevation. Maps of apparent magnetisation, free from topographic effects, may then be used for interpretation, in particular to identify areas of abnormal near surface magnetisation. This method was used to analyse draped data from White Island, an active andesitic volcano in the Bay of Plenty, New Zealand. The analysis showed that although the magnetic data are dominated by topographic effects, several areas of abnormally high near-surface magnetisation are present, including two areas that appear to be associated with the active vent. Subsequent surface sampling has confirmed the presence of highly magnetised rocks in some of these areas. High magnetisation areas in the vicinity of the active vent are interpreted to be zones of magnetite that stabilized below the Curie temperature. The discovery of ejecta containing secondary magnetite supports this interpretation. Model calculations, using a topographic model with magnetisations derived from the inversion, demonstrate that the areas of anomalous magnetisation would not have been resolved in a constant-elevation survey. Magnetic data from an area in the Taupo Volcanic Zone, New Zealand, which is geologically covered by Recent acid volcanics (mainly rhyolites), were also analysed. For this case a digital terrain model was derived directly from the barometric and radar altimeter data recorded during the survey. Apparent magnetisations calculated using a shell model proved unrealistically high. However, a flat-based model of the topography gave apparent magnetisations that were within the range measured for other Taupo Volcanic Zone rhyolites.